I don't know when it started, but I noticed recently that nautilus (3.4.2) doesn't show thumbnails anymore. It doesn't depend on the file type... images, videos, text, no matter what. No thumbnails, no preview.

I don't see the generic filetype-icons that you see when you turn thumbnails off at all. Instead, I see the icons that indicate that thumbnails are being created/loaded.

I tried to delete ~/.thumbnails/ and watched as it was re-created and filled with beautiful preview images next time I logged in. So the thumbnails are being created, they're just not read by nautilus.

I have this same problem. I even rebuilt my system from scratch to see if it would matter, but still no thumbnails in nautilus._________________Life is too short and too important to { take seriously | use Windows }.

I don't know if this will help you, but I found that I had the following lines in my package.keywords file:
--snip--

Mhm... well, yes, I had them there, too.... but trying to remove them initiated a cascade of fowngrades that probably would have brought me away from Gnome 3.4. (There was a reason why I had them in package.keywords.)

So I went the other way and installed Gnome 3.6.

I have previews now, at least for images and some (not all) video files.